my guess is that because they have sold so many S3/S4's, there are bound to be alot of reports of people with charging port problems. Many people are hard on their phones and use them alot while they are charging, things like that.
Its the same thing as the rash of broken dc jack's on laptops, you get young people that don't realize how fragile it is and use it while its plugged in. In a device like galaxy s4, it likely uses alot of battery which means wear and tear and abuse on the micro usb port.
Incidentally I was trying to explain to my dad how the wireless charging on his nexus 4 would help prolong the life of the usb port. He is the kind of guy that breaks things like that. He thought it made sense but he won't buy a wireless charger. Oh well.